In thin film technology there has always been a tradeoff between the material quality of the film and the ease of depositing that thin film. Epitaxial films represent the highest level of quality, but they must be grown on and are accompanied by cumbersome, expensive, bulk single crystal wafer substrates. For some time, research has focused on the possibility of creating epitaxial quality thin films on arbitrary substrates while maintaining the ultimate in crystalline perfection.
The main approach has been to attempt to reuse the substrate wafer by separating it from the epitaxially grown film. However, to undercut a very thin film over its entire area without adversely affecting the film or the underlying substrate, the selectivity must be extremely high. This is very difficult to achieve. For example, J. C. Fan [see J. C. Fan, J. Phys. (parts) 43. 1-327 (1982)] has described a process in which an epitaxial film is cleaved away from the substrate on which it is grown. Such cleavage, at best, is difficult to achieve without damage to the film and/or substrate, or without removal of part of the substrate. Also, in some instances, the cleavage plane (&lt;110&gt;) and the growth plane (&lt;100&gt;) of the film may be mutually exclusive.
In a paper by Konagai et.al. [see: J. of Crystal Growth 45, 277-280 (1978)], it was shown that a Zn doped p-Ga.sub.1-x Al.sub.x Ax layer can be selectively etched from GaAs with Hydrofluoric acid (HF). This observation was employed in the production of thin film solar cells by a number of techniques. In one technique, zinc doped p-Ga.sub.1-x Al.sub.x As was grown by liquid phase epitaxy (LPE) on a n-GaAs grown layer on a GaAs single crystal substrate. During this LPE growth of the Zn Ga.sub.1-x Al.sub.x As.sub.x, Zn diffuses into the surface of the underlying GaAs to form a p-type GaAs layer and hence p-n GaAs junction. The surface p-Ga.sub.1-x Al.sub.x As is then selectively etched away leaving the p-n junction GaAs layers on the GaAs substrate.
In another process for fabricating solar cells, Konagai et.al. (op.cit.) describe a "peeled film technology". Here, a 5 micron thick Ga.sub.0.3 Al.sub.0.7 As film is epitaxially grown on a GaAs &lt;111&gt;substrate by LPE. A 30 micron thick Sn doped n-GaAs layer is then grown over the Ga.sub.O.3 Al.sub.0.7 As layer and a p-n junction is formed by diffusing Zn into the specimen utilizing ZnAs.sub.2 as the source of Zn. Appropriate electrical contacts are then formed on the films using known photoresist, etch and plating techniques The surface layer is then covered with a black wax film support layer and the wafer is soaked in an aqueous HF etchant solution. The etchants selectively dissolves the Ga.sub.0.3 Al.sub.0.7 As layer which lies between the thin solar cell p-n junction device layers and underlying substrate, allowing the solar cell attached to the wax to be peeled off the GaAs substrate for placement on an aluminum substrate. The wax provides support for the peeled film prior to attachment to the new substrate.
While the technique described above has been described in the literature for about twelve years, it was not adopted by the industry. One reason for shunning the technique was the difficulty encountered in completely undercutting the Ga.sub.0.3 Al.sub.0.7 As `release` layer in a reasonable time, especially when the area of the film to be peeled was large. This difficulty was thought to be due to the formation and entrapment of gas, formed as a reaction product of the etching process within the etched channel. The gas could create a bubble in the channel, for example, thereby preventing or diminishing further etching and causing cracking in the epitaxial film. The problem could only be partially obviated by using very slow reaction rates (very dilute HF solutions). Since the time required for peel-off, and the need to ensure that no change, or at least minimal change, to the overlying film is important, the process was virtually abandoned.
A means for providing for the needed circulation of etchant and reaction products and the release of any gaseous reaction products of the etching process while maintaining high selectivity is therefore desired.
One such attempt to resolve the prior art problems is described by T. J. Gmitter et.al. in U.S. Pat. No. 4,846,931 who developed a process which employs a single crystal semiconductor substrate upon which has been grown, by some standard process such as MBE, MOCVD, VPE, and the like, an epitaxial ("epi") layer in which electronic, optical or other devices have been or will be made. A key part of the epi layer, however, is that at the bottom of the epi layer which is to be removed from the substrate, a "release layer" of semiconductor is grown or otherwise introduced. This release layer is thin (on the order of tens or hundreds of angstroms) and will later be etched away in an acid or other solution, thus undercutting the overlaid epi to ultimately separate it from the substrate upon which it was grown. Next, Gmitter et.al. apply a polymeric tension/support layer comprising a black wax/solvent mixture. Two primary functions are attributed by Gmitter et.al. to this layer. The first is that it is necessary for the polymeric layer to be in tension on the epi layer (so that the epi layer is in compression) whereupon the polymer/epi combination will curl up at its edges while the release layer is dissolved by the etch solution. It is asserted that this curling is essential in order for the reaction products of the etching, that is, the evolved gas, to be removed from the etched area through the thin channel opening of the release layer without blocking the progress of the etch. The second function claimed for the polymeric layer is that it provides mechanical support for the very thin and fragile epi layer once it is removed from the underlying substrate on which it was grown. After applying the polymeric tension/support layer Gmitter et.al. immerse the polymer/epi/substrate combination into an appropriate etch solution which dissolves the release layer and allows the epi/polymer combination to float free of the substrate. The freed epi layer is then adhered to a new substrate by using van der Waal's forces or an appropriate glue. Gmitter et.al. complete the process by removing the polymeric tension/support layer whereupon the epi layer is left attached to the new substrate.
Significance of the teaching of Gmitter et.al. lies in their demonstration that different semiconductor technologies can be combined on a common substrate; that semiconductor devices can be stacked in three dimensions to produce higher levels of integration; and that semiconductor devices can be freed from the substrate upon which they were grown (which substrate can posses undesirable properties) and placed on new substrates upon which they could not originally have been grown but which have more desirable properties. In addition, the substrates upon which the epi layers were originally grown can be reused to grow new epi layers, saving the expense of acquiring new substrates for future growths.
Notwithstanding the significant advances brought to semiconductor processing and device technology by Gmitter et.al., two very important problems were left unresolved namely, the procedure (a) does not lend itself well to the mass transfer of lifted-off epi layers to a new substrate; and (b) does not permit the lifted-off layers to be readily aligned with other devices or features on the new substrate. These two problems are critical because they limit the practicality of Gmitter et.al. lift-off on the large scale necessary for industrial implementation.
There are other less important, albeit significant, problems with the Gmitter et.al. procedure such as: the polymeric layer is somewhat fragile and per se flexible which creates difficulties in handling epi layers attached to it and in getting those epi layers to lie flat on the new substrate after transfer; very small pieces of epi are difficult to handle and transfer; very large epi layers, that is, those approaching the size of an entire 2'' or 3'' wafer become impractical to lift-off because the long etch time, (i.e., days), required to undercut the epi completely from the edge and the difficulty in handling the resulting large and fairly delicate polymer/epi combination after lift-off. These problems have further prevented Gmitter et.al. from being accepted for use in a production environment and accentuate the need toward which the present invention is directed.